Add rule to allow agent role to curl /metrics/slis

[jennchenn]

What this PR does / why we need it:

A rule was added to allow the agent to curl /metrics/slis, exposing the SLI metrics provided by Kubernetes as a part of the changes made in DataDog/integrations-core#15731

Special notes for your reviewer:

Does the corresponding change need to be made in the operator as well?

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• Chart Version bumped
• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)

[L3n41c]

Does the corresponding change need to be made in the operator as well?

Indeed, this change will have to be ported to the datadog-operator.
Here is where the datadog-operator is defining the agent cluster role:

• https://github.com/DataDog/datadog-operator/blob/480f69931540a7de1808ffd7bd9b5043c1249fe7/controllers/datadogagent/component/agent/rbac.go#L32-L37
• https://github.com/DataDog/datadog-operator/blob/480f69931540a7de1808ffd7bd9b5043c1249fe7/pkg/kubernetes/rbac/const.go#L87

But, in order for the operator to be able to grant some permissions, it needs to have them for itself. Those are defined in:

• https://github.com/DataDog/datadog-operator/blob/480f69931540a7de1808ffd7bd9b5043c1249fe7/config/rbac/role.yaml#L9-L12